Syndiotactic dyads

Consider the description of the sequence distribution of isotactic and syndiotactic placements in the polymerization of a monosubstituted ethylene. The approach is general and can be applied with appropriate modification to the 1,4-polymerization of a 1,3-diene. Dyad tac-ticity is defined as the fractions of pairs of adjacent repeating units that are isotactic or syndiotactic to one another. The isotactic and syndiotactic dyads (XV) are usually referred to as meso and racemic dyads. The horizontal line in XV represents a segment of the polymer... 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

The 100 MHz 1H—NMR spectra in chloroform-d afforded the following information. Isotactic polymers prepared by zinc catalyst and atactic polymers by KOH catalyst had a common feature in having a sharp, symmetrical AB-type quartet at 6.43,6.53,6.60, and 6.70 ppm. Therefore, this quartet could be assigned to a head-to-tail dyad, because the latter polymer was known to contain no head-to-head and tail-to-tail linkage. Thus, in this spectrum the absorptions originating from isotactic and syndiotactic dyads overlap. 

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

Comparison to the model spectra strongly suggests that in the CF2 resonance region of the polymer spectrum, the peak at 104.2 represents CFa groups which are between monomer units in isotactic dyads, — dd or ll —, whereas the peak at 106.0 represents CFa groups which are between syndiotactic dyads, dl or Id. As before, the first type of CFa group may be designated as meso and the second as racemic. If the chain were random or atactic , these peaks would be of equal... 

Signals are split into two closely located singlets and comparison of the C-NMR spectrum of the racemic polymer, obtained from the racemic monomer, with that of the optically active polymer (prepared from (+)-( R, 5S)-6,8-dioxabicyclo[3,2,l]oc-tane indicate, that the lower field, smaller peak of each signal pair comes from the D-L (syndiotactic) dyad. The hi er field signal can thus be ascribed to the dyad structures of D-D and L-L consecutive units (isotactic dyads). 

The corresponding 13C-NMR spectrum is shown in Fig. 8.3. One set of lines (more abundant) is identical with the spectrum of the polymer obtained from the optically pure enantiomer. Thus it was concluded that in the polymer prepared from racemic DBO two sets of signals corresponding to isotatic and syndiotactic dyads are present. 

Since P and Pr are synonymous with isotactic and syndiotactic dyad fractions m and r, respectively, probabilities for isotactic, syndiotactic, and heterotactic triad sequences will be given by... 

An explanation for the preference for syndiotacticity during MMA polymerization was proposed by I suruta et al. I hey considered that the propagating radical should exist in one of two conformations and showed, with models, that attack on the less hindered side of the preferred conformation (where steric interactions between the substituent groups are minimized) would lead to formation of a syndiotactic dyad while similar attack on the less stable conformation would lead to an isotactic dyad,... 

There are a few exceptions to this general rule. One of the few examples of an effect on polymer stereochemistry was provided by Dais et who found that polymerization of 31 above the cmc initiated by y-irradiation at 25 C yields polymer composed entirely of syndiotactic dyads P m) =0. When the double bond was distant from the polar head group in 32, the tacticity observed was similar to that observed in solution polymerization 18. Polymerization of 31 at... 

It is clear that a determination of any two triad fractions allows a complete definition of both the triad and dyad structures of a polymer via Eqs. 8-10 through 8-13. An atactic polymer is one in which (r) = (m) = 0.5 and mm) = (rr) = 0.25, mr) = 0.5 with a random distribution of dyads and triads. The (aU-) isotactic polymer has (w) = mm) = 1. The (all-) syndiotactic polymer is defined by (r) = (rr) — 1. For random distribution with (m) 7 (r) 7 0.5 or mm) / rr) / 0.25, one has different degrees of syndiotacticity or isotacticity. Isotacticity predominates when (m)>0.5 and mm) >0.25 and syndiotacticity predominates when (r)>0.5 and (rr)>0.25. These pol3miers are random tactic polymers, containing random placement of isotactic and syndiotactic dyads and triads. When the distribution of dyads and triads is less than completely random, the polymer is a stereoblock polymer in which there are block (which may be short or long) of isotactic and syndiotactic dyads and triads. 

Catalyst Monomer Isotactic dyads (%) Syndiotactic dyads (%) Reference... 

Tacticity Parameter as a Function of Polymerization Temperature of PVC where Represents the Fraction of Syndiotactic Dyads. 

Figure 1.17 The basic tactic structures of a vinyl polymer, (a) Isotactic (b) syndiotactic (c) atactic. Methylene protons are equivalent in syndiotactic dyads but not in isotactic dyads.

Figure 2. When the classical WCl6/Me4Sn was used as initiator an atactic microstructure was obtained whereas initiation with the Schrock molybdenum initiator, Mo(=CHR)(OR )2(=NAr), where R is tertiary butyl, gave a thermo plastic with a Tg of 92°C and a microstructure consisting of >98% trans vinylenes and >92% syndiotactic dyads.

NMR m and r dyad analysis of syndiotactic polystyrenes prepared with MAO activated CpTiCb or Cp TiCl3 systems was in agreement with the Bernoullian symmetric statistical model for stereoselective propagation. The polymer microstracture is a long sequence of syndiotactic dyads with only isolated m defects and no consecutive isotactic dyads (Figure 14.17), which is consistent with a chain-end control mechanism directed by the configuration of the tertiary carbon of the last inserted styrene unit (unlike 1,3-asymmetric induction). Thus, the syndiotactic preference exerted by the chain end arises from the phenyl-phenyl repulsive interaction between the last inserted... 

The monomer 7-methylnorbornadiene (7-MeNBD) also undergoes polymerization in the presence of several conventional metathesis catalysts to form poly(7-MeNBD). It is noteworthy that the catalyst OSCI3 (in 1 1 EtOH/PhCl) leads to polymer stereochemistry that is significantly different from that of poly(fl fi-7-MeNB). The OsC -derived polymer of the monoolefin contains predominantly atactic trans olefin structures, whereas the analogous diolefin polymer is composed of nearly exclusively cis alkene structures (97%) and predominantly syndiotactic dyads (r m = 75 25 through the NMR signals of the methyl substituent at 8 16.2-17.0 ppm)." The catalysts Reds... 

Furukawa et al. explored the use of methanol and ethanol as additives for diethylzinc-based epoxide polymerization systems, and found that both the yield and crystallinity of the resulting polymers were inferior to those for polymers synthesized with the ZnEt2/H20 system. The use of achiral alcohols as cocatalysts was revisited in 1994 when Kuran and Listos reported the polymerization of propylene oxide and cyclohexene oxide (a meso molecule) with ZnEt2/polyhydric phenol (such as 4-tert-butyl-catechol), phenol, or l-phenoxy-2-propanol. The poly(propylene oxide) formed from these systems contained mostly isotactic dyads (72% m), whereas the poly(cyclohexene oxide) contained mostly syndiotactic dyads (80% r) (Scheme 24.8). 

Stereosequences in PLAs are routinely determined by the examination of nuclear magnetic resonance (NMR) spectra, particularly with regard to the NMR methine resonance and the proton decoupled NMR methine resonance. Figure 25.4 shows the possible PLA tetrad stereosequences (/ = isotactic dyad = syndiotactic dyad). The original ( C and H) NMR peak assignments for tactic PLAs were put forward at the tetrad level of sensitivity by Kricheldorf et al. based... 

FIGURE 25.4 The possible stereosequences in PLA at the tetrad level (i = isotactic dyad = syndiotactic dyad). 

---